1. Field of the Invention
The present invention relates to an apparatus and method for measuring characteristics of light, and more particularly to an apparatus and method for measuring the phase of an optical signal or for concurrently measuring the amplitude and phase of an optical signal.
2. Description of the Related Art
Conventionally, in order to measure the phase of an optical signal, there has been used a homodyne phase detection method and a method in which phase detection is performed after heterodyne detection. However, since both methods utilize an interferometer, the interferometer must be stabilized.
Since such an interferometer is generally stabilized by control the phase variation (variation in optical path difference), it is difficult to separate a change in the phase of a signal to be measured from a phase variation of the interferometer.
Also, there has been employed a method in which two light sources emitting two optical signals of different wavelengths are used in order to detect variations of the interferometer. However, this method makes the structure of a measurement apparatus complex. In addition, it is difficult to cause optical paths within the interferometer to coincide with each other completely, and complicated processing is needed to extract a target phase signal from two detected phase signals.
Since the above-described conventional techniques require stabilization of the interferometer or disposition of two light sources emitting two optical signals of different wavelengths, the conventional techniques still have room for improvement.
Meanwhile, in order to evaluate a surface or interface of a nonlinear optical crystal or substance, there has been measured a second harmonic that is generated upon radiation of a fundamental wave onto the surface or interface, or an optical signal that is generated upon radiation of two fundamental waves onto the surface or interface and has a frequency equal to the sun or difference of the frequencies of the two fundamental waves. Since optical signals to be measured are considerably weak as compared with the fundamental wave or waves, design of a separation circuit for separating the optical signals from the fundamental-wave component or components is difficult. If components other than an optical signal to be measured leak from the separation circuit, the leaked components become background light. Achieving complete separation involves a large amount of loss, and deteriorates measurement sensitivity. As mentioned above, homodyne detection and heterodyne detection have been used in order to measure the amplitude and phase of an optical signal with high sensitivity. Since both the homodyne detection and heterodyne detection utilize the effect of interference between signal light and local oscillator light, effects of background light can be eliminated. However, these methods utilize an interferometer, and the interferometer must be stabilized. Since such an interferometer is generally stabilized by control the phase variation, measuring the amplitude and phase of an optical signal with high sensitivity is impossible, unless the phase variation of the interferometer is separated from the phase variation of the optical signal to be measured.
Since the above-described conventional method requires stabilization of an interferometer in order to achieve highly sensitive measurement of the amplitude and phase of an optical signal through homodyne detection or heterodyne detection, there is still room for improvement.